Many systems use batteries each formed of a battery pack or battery array, including a plurality of battery cells connected in series to each other.
When such battery cells are charged to voltages significantly higher than voltages within a rated charge range or discharged to voltages lower than voltages within a rated charge range, they may be dangerous.
The imbalance between the charged states of battery cells is caused by various factors, and occurs during the manufacture of batteries or the charge/discharge of batteries. In the case of lithium ion cells, the manufacture of cells is strictly controlled in a factory to minimize the differences between the capacities of the cells of a battery array. However, imbalance or inequality between cells may occur due to various factors, regardless of the states of the cells, in which balance or equality was achieved in a factory after the cells were initially manufactured.
The factors influencing the imbalance of cells may include, for example, the chemical reactions, impedances and self-discharge rates of respective cells, the reduction of the capacities of the cells, variation in the operating temperatures of the cells, and different types of variation between the cells.
The inconsistency between the temperatures of cells is an important factor responsible for causing imbalance in cells. For example, “self-discharge” is caused in a battery cell, and is a function of battery temperature. A battery having a high temperature typically has a self-discharge rate higher than that of a battery having a low temperature. As a result, the battery having a high temperature exhibits a lower charged state than the battery having a low temperature over time.
Imbalance is a very series problem in the charged state of a battery. For example, the ability of a battery to supply energy is limited by a battery cell having the lowest charged state, which may typically occur in electric vehicles.
If the battery cell is fully consumed, other battery cells lose the ability to continue to supply energy. This is the same even if the other battery cells of the battery still have the ability to supply power. Therefore, an imbalance in the charged state of battery cells reduces the power supply ability of the battery.
Of course, the above description does not mean that, when one or more battery cells are consumed, the supply of power by the remaining battery cells is completely impossible. However, it means that, in the case of series connection, even if one or more battery cells are fully consumed, the battery can be continuously used as long as charge remains in the remaining battery cells, but, in that case, voltage having a reversed polarity is generated in the battery cell for which discharge has been completed, with the result that the battery cell may be in danger of explosion due to the overheating thereof or the generation of gas, and thus the battery loses power supply ability.
Various methods of correcting the imbalance between the charged states of battery cells have been proposed, and one of the methods is shown in FIG. 1.
FIG. 1 is a diagram showing a prior art centralized charge equalization apparatus.
Referring to FIG. 1, the prior art centralized charge equalization apparatus includes a transformer T, N semiconductor switching elements D1 to Dn, a control switch SW, and a voltage detection and drive signal generation unit 10.
The transformer T includes one primary winding and N secondary windings, the N secondary windings being bound on one common core, and the primary windings and the secondary windings having opposite polarities. In other words, the dot of the primary winding and the dots of the secondary windings are located on different sides. The secondary windings of the transformer T have the same number of turns, and the turns ratio of the primary winding to the secondary windings is N1:N2.
The semiconductor switching elements D1 to Dn are each connected between one end of each of the secondary windings and the positive (+) electrode of each of the batteries B1 to Bn, and rectifies energy that is supplied from each of the secondary windings to each of the batteries B1 to Bn.
The control switch SW is connected in series to the primary windings, and forms a closed circuit in response to a drive signal from the voltage detection and drive signal generation unit 10.
The voltage detection and drive signal generation unit 10 detects respective voltages of the series-connected batteries B1 to Bn, compares the detected voltages with a reference voltage, and generates a drive signal for discharging energy from batteries having voltages higher than the reference voltage, that is, overcharged batteries.
A charge equalization method for the prior art centralized charge equalization apparatus is described below.
First, the voltage detection and drive signal generation unit 10 detects respective voltages of the N series-connected batteries B1 to Bn.
Thereafter, the voltage detection and drive signal generation unit 10 turns on the control switch SW if the voltage of any one of the N series-connected batteries B1 to Bn is higher than a reference voltage.
Accordingly, energy from the N series-connected batteries B1 to Bn is converted into magnetic energy, and is stored in the transformer T of the primary windings.
Thereafter, when the voltage detection and drive signal generation unit 10 turns off the control switch SW, the magnetic energy stored in the primary windings of the transformer T is converted into a charge, and is stored in the N series-connected batteries B1 to Bn via the secondary windings and the semiconductor switching elements D1 to Dn.
In this case, greater charges move to batteries having lower electric potentials via the secondary windings bound on the common core of the transformer T while the control switch SW is turned off, thereby equalizing charges.
However, the prior art centralized charge equalization apparatus has a problem in that it is difficult to fabricate the secondary windings of the transformer T because a number of secondary windings equal to the number of batteries is bound on one common core, so that a number of secondary windings equal to the increased number of series-connected batteries must be bound to one common core.
Furthermore, the prior art centralized charge equalization apparatus has a problem in that the turns ratio of the primary winding to the secondary windings of the transformer T increases in proportion to the number of series-connected batteries, so that it becomes difficult to fabricate primary windings in proportion to the increase in the number of batteries.